The formulation of distillate fuels for internal combustion engines has become increasingly sophisticated and complex. Basic diesel fuels are tailored through additives aimed to reduce fuel hazing, particulate and gaseous emissions, inhibit corrosion, reduce deposits and more pertinent hereto, improve lubricity. Driven by demanding regulatory requirements in the U.S. and Europe, increasingly severe specifications have been imposed to diesel fuels, particularly with respect to sulfur content and in some areas aromatic content. For example, in 1991, clean burn, Class 1 diesel fuels were introduced in Sweden; these fuels contain less than 10 ppm sulfur and less than 5% vol. aromatics. In the United States, the Environmental Protection Agency promulgated a regulatory sulfur content in diesel fuels which was limited to 0.05% wt. commencing in 1993. Similar reductions in sulfur will occur in Japan in 1997.
Removal of sulfur compounds and hydrotreating of distillate fuels, in combination with increasing injection pressures in fuel systems in modern engines, have caused concerns over lack of fuel lubricity. This could lead to problems of excessive wear of fuel-lubricated components such as fuel pumps, fuel injectors, etc. The present invention provides a distillate fuel additive which exhibits improved lubricity, and wear and frictional performance.
Esters have generally excellent thermal and oxidative stability characteristics, and have been widely used in synthetic or partially synthetic crankcase lubricants. The art has recently recognized the potential role esters may serve as fuel additives. For example U.S. Pat. No. 5,366,519 discloses the use of certain polyoxyalkylene hydroxylaromatic esters as fuels additives, including diesel fuels, to reduce engine deposits.
The prior art also teaches that high molecular weight esters may survive the combustion in the cylinder and thereby be available to provide surficial lubricant benefit to the cylinder walls and piston rings while low molecular weight esters provide detergency benefits such as reduced injector deposits. U.S. Pat. No. 4,920,691 teaches a combination of a low molecular weight straight chain carboxylic acid ester, i.e., molecular weight less than 200, and a high molecular weight straight chain carboxylic acid ester, i.e., molecular weight ranging from 300 to 1000 to achieve both detergency benefits and cylinder wall lubrication. In addition to increasing the cost of the fuel, it has been recognized that the amount of detergent additives need be minimized because of the deleterious effects the by-products of such additives have on crankcase lubricants; see, for example, U.S. Pat. No. 5,004,478. Small amounts of the by-product of these additives, upon breakdown in the combustion chamber, wind up in the crankcase lubricant and contribute to engine oil breakdown.